I think the main difference is that in CSGO (and Dota) cosmetics sold directly by Valve in the game can be regulated to follow the law in all countries its available in (IE, the changes made for France in these games). Valve does not have as much control over 3rd party servers and sites. If its entirely separate like more CS gambling sites, they can (and do) just distance themselves from it and ignore it in all but the most extreme cases. On servers and custom games though, its accessed directly through their game, which is why I can see them being more strict with it.
I think the main difference is that in CSGO (and Dota) cosmetics sold directly by Valve in the game can be regulated to follow the law in all countries its available in (IE, the changes made for France in these games). Valve does not have as much control over 3rd party servers and sites. If its entirely separate like more CS gambling sites, they can (and do) just distance themselves from it and ignore it in all but the most extreme cases. On servers and custom games though, its accessed directly through their game, which is why I can see them being more strict with it.